This invention relates to the use of a polysaccharide known as chitin, as well as other derivatives of chitin, and copolymers of the family of materials known as amino-polysaccharides useful for making contact lenses or parts thereof, artificial corneas and interocular lens types, drug delivery systems, vaginal spermicides and creams, injectable encapsulation materials and other medical devices and pharmaceuticals. These amino-polysaccharides are used alone or as components in a polymeric arrangement, such as polymer blends, graft or block copolymers, or any other combination of modified compositions.
Chitin is an amino sugar, in which one or more of the hydroxyl groups of the carbohydrate are replaced by an amino group. Chitin is one of the most abundant amino sugar derivatives. The amino polysaccharide is made up of 2-acetamido-2-deoxyglucose units linked in a B-1,4 manner similar to that in cellulose, or starch, or in general it is indicated as N-acetyl-D-glucosamine, and is found as a structural material in the invertebrate animal world. The exoskeletons of insects and crustaceans contain large amounts of this amino polysaccharide. The observation of the exoskeleton of shrimp and other invertebrates indicates that this material can be made clear, flexible, hard, and permeable, which would be useful for medical devices.
Chitin has been estimated to be the second most abundant polysaccharide in nature with synthesis in the neighborhood of a billion tons a year. However, in the natural state it occurs in small flakes or as short fibrous materials, and is not capable of forming useful shaped articles without solution and reprecipitation or renaturing. Methods of dissolving chitin in certain solvents are described in the literature. For example, Clarke and Smith, J. Phys. Chem., 40, 863 (1936), use aqueous acids or lithium salts for solution and regeneration of chitin. These authors observed the formation of addition compounds of chitin with lithium thiocyanate and with sodium hydroxide under certain conditions. However, the formation of addition compounds or complexes of chitin with organic compounds has not been described.
The N-acetyl-D-glucosamine can be prepared in 60 to 70% yields by the hydrolysis of the exoskeleton of crustaceans with concentrated hydrochloric acid; ##STR1##
Poly(N-acetyl-D-glucosamine) is a major component of naturally occurring chitin. The naturally occurring material has not only the poly(N-acetyl-D-glycosamine) but also inorganic salts thought to be forms of calcium carbonate and proteinaceous materials, the composition of which is not presently known. The term N-acetyl-D-glucosamine is used herein to refer to the various naturally occurring forms of chitin including the purified and or naturally occurring form. A method of extracting the N-acetyl-D-glucosamine is the following: chitin is finely ground in a ball mill overnight or until it passes a 6 mm screen and can be retained by a 1 mm screen. 149 g of this finely ground material is decalcified by extracting with 825 ml of 2 N HCl at 4.degree. C. for 48 hours in a flask stirred with a magnetic stirrer. The material is collected by centrifugation and washed repeatedly with water until neutral. The ash content is about 0.4-0.5%. The decalcified chitin is then stirred at room temperature with 1500 ml of 90% formic acid overnight. The mixture is centrifuged and the residue repeatedly washed with water. The washed chitin is then suspended in 2 liters of 10% NaOH solution and heated at 90.degree.-100.degree. C. for 2.5 hours. The solution is filtered, and washed with water until neutral, washed several times with absolute ethanol and ether, and dried at 40.degree. C. under reduced pressure to yield poly(N-acetyl-D-glucosamine).